Among the known types of apparatuses for applying an electrical stimulation to a patient is the interference type apparatus that is used to stimulate structures located within the patient's body, such as muscles and/or the nerves that control muscle action, that are reached with relatively high frequency signals, but are responsive to relatively low frequency signals. The interference apparatus operates by applying two primary signals of relatively high, but slightly different, frequencies to the patient's body. The primary signals, due to their relatively high frequency, penetrate the patient's body and reach the aforementioned structures where they intersect and produce a beat signal having a relatively low frequency that is equal to the slight difference in the frequencies of the primary signals. Exemplary of known interference type apparatuses is U.S. Pat. No. 4,374,524 to Hudek (1983) which illustrates the use of a square-wave signal generator in conjunction with a plurality of phase-locked loops and low-pass filters to produce a plurality of sine-wave, primary signals. Also representative of known interference type apparatuses are U.S. Pat. No. 4,071,033 to Nawracaj et al. (1978), and U.S Pat. No. 4,153,061 to Nemec (1979) which, in addition to providing two primary signals of different frequencies, also amplitude modulate the primary signals to achieve various therapeutic effects. For example, in Nawracaj two square-wave, primary signals are amplitude modulated by either a square-wave, ramp, exponential, semi-sine or sine-wave signal. In Nemec two sine-wave, primary signals are modulated by two low-frequency sine-wave signals to achieve stimulation at the point of application to the patient's body in addition to producing a beat signal therein.
Another known type of apparatus for applying an electrical stimulation to a patient's body is exemplified in U.S. Pat. No. 4,392,496 to Stenton (1983). Stenton applies two, apparently, square-wave signals to a patient's body in an alternating fashion to achieve muscle stimulation and prevent disuse atrophy. Further, in order to achieve optimal muscle stimulation and enhance the comfort of the patient, Stenton provides for the adjustment of several parameters associated with the applied signals, such as amplitude and frequency.
Yet another apparatus for administering an electrical stimulation to a patient's body is illustrated in U.S. Pat. No. 4,580,570 to Sarrell et al. (1986). The method of Sarrell is characterized by the application of pulses that have a relatively high voltage, high peak but low average current, and short duration. Moreover, the apparatus of Sarrell can be adjusted to apply the aforementioned pulses continuously, periodically, or in an alternating fashion.
Typically, the patient's body produces electrical signals, in the form of sensory and muscle nerve impulses, that are exponential in character. Characteristic, however, of the foregoing apparatuses is the application of signals, like sine-waves and square-waves, that are alien to the typical patient's body. Consequently, the patient can experience a certain amount of discomfort. Moreover, exposure to such alien signals can traumatize certain biological structures associated with the patient. Consequently, there exists a need for an electrotherapeutic device that produces a signal or signals that more closely resemble the exponential character of the patient's natural signals. There also exists a need for an electrotherapeutic device that produces a signal capable of penetrating the patient's body and that reduces any trauma imposed upon biological structures associated with the patient's body.